Protein kinases (PKs) are enzymes that catalyze the phosphorylation of hydroxyl groups on tyrosine, serine or threonine residues of proteins. Protein kinases act primarily as growth factor receptors and play a central role in signal transduction pathways regulating a number of cellular functions, such as cell cycle, cell growth, cell differentiation and cell death.
One important signal transduction pathway is the mitogen-activated protein kinase (MAPK) pathway. The MAPK signaling pathway is responsible for the regulation of cell growth, differentiation, proliferation and survival and its dysregulation is implicated in a broad spectrum of cancer. (Hoshino, et al., Oncogene, 1999, 18, 813-822)
The MAPK signaling pathway is one of multiple signaling pathways activated by GTP-bound RAS. Initially, extracellular stimuli such as mitogens, hormones or neurotransmitters induce receptor tyrosine kinase dimerization leading to increased levels of GTP-bound RAS. Activated RAS recruits dimerized RAF kinase to the plasma membrane whereby RAF is activated by autophosphorylation or phosphorylation by other kinases. The activation of RAF initiates the phosphorylation cascade down the MEK/ERK pathway, in which activated RAF phosphorylates and activates MEK1/2 which in turn phosphorylates and activates ERK (or extracellular signal-regulated kinase, also called p44/42 MAPK) which in turn phosphorylates a number of targets including nuclear transcription factors that lead to changes in gene expression.
RAF is a family of serine/threonine kinases comprising three isoforms called ARAF, BRAF and CRAF (also called raf-1). BRAF is currently a cancer therapeutic target, as mutations in the BRAF gene are among the most common in cancer (Haluska, et al., Clin Cancer Res 2006, 12(7 Pt 2), 2301s-2307s; Ikediobi, et al., Mol. Cancer Ther. 2006 5(11), 2606-2612; Greenman, et al., Nature 2007 226(7132), 153-158). The majority of mutant BRAF have been found to exhibit elevated kinase activity as measured by levels of phosphorylated MEK or ERK found endogenously in COS cells (Wan et al. Cell 2004 116, 855-867). BRAF mutations have been identified in about 7% of all known cancers, including 27-70% of melanoma (Davies et al. Nature, 2002 417, 949-954), 42-50% of papillary thyroid carcinoma, 36-53% colorectal cancers, and 5-22% serous ovarian cancers and to a lesser extent in breast cancer, endometrial cancer, liver cancer, sarcoma, stomach cancer, Barret's adenocarcinoma, gliomas including ependymomas and lung cancer including 1-2% of non small cell lung cancer (See Davies et al. Nature, 2002, 417, 949-954; Garnett and Marais, Cancer Cell, 2004 6, 313-319; Ouyang et al. Clin Cancer Res 2006 12(6), 1785-1793; Melillo, et al., J. Clin. Invest. 2005, 115, 1068-1081; Wilhelm, et al., Nat. Rev. Drug Discov., 2006 5, 835-844; and Ji et al. Cancer Res 2007 67(10), 4933-4939). Over forty different missense mutations of BRAF have been identified, but among them, the V600E mutation, has been found to be the most predominant (Fecher, et al., J. Clin. Oncology 2007, 25(12), 1606-1620), accounting for nearly 90% of the mutations in melanoma and thyroid cancer and for a high proportion in colorectal cancer, which makes this mutation a particularly attractive target for molecular therapy. A study of the crystal structures of both wild type and V600 mutants suggests that substitution at the 600 position destabilizes the inactive conformation of the enzyme (Wan et al. op cit.). However, V600E mutation is comparatively rare in non-small cell lung cancer, which is more likely than not to be associated with non-V600E BRAF missense mutations (Brose et al. Cancer Res., 2002 62, 6997-7000). Other non-V600E BRAF missense mutations are also implicated in melanoma, breast cancer, lung cancer, colorectal cancer, liver cancer, ovarian cancer, leukemia including acute lymphoblastic leukemia (ALL), non-Hodgkin's lymphoma, Barret's adenocarcinoma, endometrial cancer, liver cancer, stomach cancer, thyroid cancer and endometrial cancer (Garnett and Marais, op. cit.).
In vivo efficacy has been demonstrated for BRAF inhibitors NVP-AAL881-NX (also AAL881) and NVP-L T613-AG-8 (LBT613) in mouse tumor xenograft models using human cell lines (See, Ouyang et al. op. cit.). Preclinical studies have also shown that BRAF inhibition by siRNA or by the small molecule RAF kinase inhibitor Sorafenib resulted in a decrease in tumor growth or metastases in animals (Sharma et al. Cancer Res., 2005, 65(6), 2412-2421; Sharma et al. Cancer Res., 2006, (66)16, 8200-8209). RAF inhibitors that have entered clinical trials include antisense oligonucleotides against CRAF such as ISIS 5132 and LErafAON and small molecule BRAF inhibitors such as BAY 43-9006 (Sorafenib), Raf-265 (formerly CHIR-265, Novartis), PLX-4032 (Plexxikon) and XL281 (Exelixis).
Although most BRAF mutations are activating mutations, mutants having impaired kinase activity have been identified, and shown to stimulate ERK activity, presumably through recruitment of CRAF (Wan op cit.). Therefore, CRAF represents another target for the treatment of diseases associated with this particular subset of BRAF mutants.
Outside of cancer, the MAPK (Raf-Mek-Erk) signaling pathway could provide targets for inflammation and inflammatory diseases. The MAPK pathway is known to control cell survival and apoptosis of inflammatory cells such as basophils, macrophages, neutrophils and monocytes (See Dong et al., Annu. Rev. Immunol., 2002, 20, 55-72; Johnson, et al., Curr. Opin. Chem. Biol., 2005, 9, 325-331; R. Herrera and J. S. Sebolt-Leopold, Trends Mol. Med., 2002, 8, S27-S3; and Kyriakis et al., Physiol. Rev., 2002, 81, 807-869). In the carrageenan-induced pleurisy rat model, it has been shown that the Erk1/2 inhibitor PD98059 inhibits eosinophilic proinflammtory cytokine release by increasing the rate of neutrophil apoptosis thereby decreasing the number of macrophage and neutrophils that perpetuate the inflammatory response (Sawatzky et al., Am J Pathol 2006, 168(1), 33-41). It is therefore possible that one downstream effect of inhibiting RAF might be the resolution of an inflammatory response and BRAF inhibitors could be useful for the treatment of inflammatory diseases or immune system disorders including inflammatory bowel disease, Crohn's disease, ulcerative colitis, systemic lupus erythematosis (SLE), rheumatoid arthritis, multiple sclerosis, thyroiditis, type 1 diabetes, sarcoidosis, psoriasis, allergic rhinitis, asthma, COPD (chronic obstructive pulmonary disease) (See Stanton et al. Dev. Biol. 2003 263, 165-175, Hofman et al. Curr. Drug Targets. Inflamm. Allergy 2004 2, 1-9).
Given the multitude of diseases attributed to the dysregulation of MAPK signaling, there is an ever-existing need to provide novel classes of compounds that are useful as inhibitors of enzymes in the MAPK signaling pathway, as discussed herein.